The manufacture of flexible heaters by photochemical etching metal foil bonded to thin, flexible electrical insulation materials is a well established practice. FIG. 1a shows metal foil 2 bonded to an electrical insulation material 9 by the use of a suitable adhesive. The resulting heater element 6 and termination pads 8 shown in FIG. 1b are formed using conventional photochemical etching techniques. Leadwires 10 (FIG. 1c) are attached to the termination pads 8 by means of crimping, welding, soldering, conductive adhesives or other joining techniques. Electrical insulation is completed by bonding suitable insulation material 12 on top of the etched heater element as shown in FIG. 1d. 
An alternative way to connect the leadwires 10 involves pre-forming openings in the top insulation layer 12 which is then bonded to the etched heater element 6. Subsequently, the termination pads 8 of the heater element 6 are connected to leadwires or connectors using crimping, welding, soldering, conductive adhesives or other joining techniques. The electrical insulation is completed by covering the heater element termination and leadwire joint with a patch of insulating material using an appropriate adhesive.
The electrical insulation materials used are in sheet form (up to 1 mm thick) and are typically non-porous. Common types of flexible insulating materials used are fibre reinforced silicone rubber, polyimide and polyester. Metal sheet (typically 10 μm–500 μm thick) is bonded to the insulating material using an adhesive. Metals and alloys used for heater elements typically have a resistivity which has a low dependence on temperature and include, for example, copper, nichrome, nickel and stainless steel. The resistance of the heater element, and consequently the operating temperature, is controlled by changing the type of metal foil, the thickness of the metal foil or the heater element design.
Other types of flexible heaters available utilise different forms of heating element and include wire-wound elements, interwoven carbon fibre sheets and metallised synthetic fibre sheets such as nickel coated polyester.
It is also known to utilise metallised fabrics and similar mesh structures in the manufacture of flexible heaters, for example as disclosed in GB 2,092,868 and DE 3210097. However such structures have total metallisation, and the electrical resistance is controlled by the metal composition, the density of application and the like.